This does not require resolving conflicts or bloating the history.

This avoids bloating the history with blank space commits.

clang-tidy would run the wrong checks/no checks at all because of a missing comma.

* CMake 3.9 is now installed by default on Travis: https://docs.travis-ci.com/user/build-environment-updates/2017-12-12/
* Clang 5.0 (and clang-format, clang-tidy) are now installed on Travis
* Just get clang-format-diff.py
* Move extensive setup to .ci/*
* Fix usage of yaml keys
* Avoid if statements by using build matrix entries
* Note clearly that one block of code will not run
* Remove comments about interleaved macOS